Well, the parts supplier I use brought in each pump, just to be sure I got the right one. We held them all up side by side and the longer one was a match for my car...which I think is 4 15/32 (basically 4.5").

So, I sat down and set my calipers to 4 15/32 and tried to figure out where that dimension comes from. And with the pump off the car and on my desk, I can't find any dimension that matches it. Because the pump is off the car, I couldn't check Amervo's option D...but it looks like that would be it. Which seems like a very weird dimension to use. Seems like you'd want to use a dimension that you could check with the pump in your hand...but hey, what do I know?

The Ames catalog shows what would be Option B in my picutre...but according to what I have here, and what came off the car, that's not the correct way to measure it. It's about a quarter inch off.

Just thought I'd update this in case anyone else was wondering.